I have a 2012 with a 351 big bore kit stock intake and stock exhaust other then a 2021 klx300r muffler. I've been running streets a lot more then dirt roads the past few years and wondering if going lidless would really help the bike with the 300r muffler or would it hurt it? I'm planning on getting the muffler insert increased from a 7/8' outlet to a 1' outlet with more of a straight shot out as now it has a few reductions in sizing on the pipe. I don't know for sure what the jetting numbers are but the bike runs very strong all over but just a tad hard to start sometimes. I know there were other threads but cant really find any close to my situation or just didn't look long enough. Also would a pre filter help with dirt and water if i do go lid less?

 

It does help for airflow and helps performance if you have jetting and exhaust mods to take advantage of the increased airflow. However, it's pretty loud, so you have to see if it's too much for your preference. For testing you can just pop it off and go for a short ride to see what you think about the noise. I kept the lid on my '06 with a 300 kit, but I opened up the factory opening a good deal more and don't run a snorkel. I do think that leaving the factory lid in place but just opening up the hole does help with splash while doing water crossings and such. The open top airbox without the lid allows water to enter easier.

 

Once you have a 1" ( or better) straight thru slipon, you are free to create/feel the power of a lidless setup.
Your CVK setup will be the usual ( as can be read below) but the main jet will need to be a DJ146 or K160 ..

 

A larger KDX200 snorkel flows more air without the extra noise. The cross section is double the stock snorkel.